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<body>
<div class="document" id="cc2420-radio-stack">
<h1 class="title">CC2420 Radio Stack</h1>
<table class="docinfo" frame="void" rules="none">
<col class="docinfo-name" />
<col class="docinfo-content" />
<tbody valign="top">
<tr class="field"><th class="docinfo-name">TEP:</th><td class="field-body">126</td>
</tr>
<tr class="field"><th class="docinfo-name">Group:</th><td class="field-body">Core Working Group</td>
</tr>
<tr class="field"><th class="docinfo-name">Type:</th><td class="field-body">Documentary</td>
</tr>
<tr><th class="docinfo-name">Status:</th>
<td>Draft</td></tr>
<tr class="field"><th class="docinfo-name">TinyOS-Version:</th><td class="field-body">2.x</td>
</tr>
<tr><th class="docinfo-name">Author:</th>
<td>David Moss, Jonathan Hui, Philip Levis, and Jung Il Choi</td></tr>
<tr class="field"><th class="docinfo-name">Draft-Created:</th><td class="field-body">5-Mar-2007</td>
</tr>
<tr class="field"><th class="docinfo-name">Draft-Version:</th><td class="field-body">1.5</td>
</tr>
<tr class="field"><th class="docinfo-name">Draft-Modified:</th><td class="field-body">2007-06-14</td>
</tr>
<tr class="field"><th class="docinfo-name">Draft-Discuss:</th><td class="field-body">TinyOS Developer List &lt;tinyos-devel at mail.millennium.berkeley.edu&gt;</td>
</tr>
</tbody>
</table>
<div class="note">
<p class="first admonition-title">Note</p>
<p class="last">This memo documents a part of TinyOS for the TinyOS Community, and
requests discussion and suggestions for improvements.  Distribution
of this memo is unlimited. This memo is in full compliance with
TEP 1.</p>
</div>
<div class="section">
<h1><a id="abstract" name="abstract">Abstract</a></h1>
<p>This TEP documents the architecture of the CC2420 low power listening
radio stack found in TinyOS 2.x.  Radio stack layers and implementation
details of the CC2420 stack will be discussed.  Readers will be better
informed about existing features, possible improvements, and limitations
of the CC2420 radio stack.  Furthermore, lessons learned from
the construction of the stack can help guide development
of future TinyOS radio stacks.</p>
</div>
<div class="section">
<h1><a id="introduction" name="introduction">1. Introduction</a></h1>
<p>The TI/Chipcon CC2420 radio is a complex device, taking care of
many of the low-level details of transmitting and receiving packets
through hardware.  Specifying the proper behavior of that hardware
requires a well defined radio stack implementation.  Although much
of the functionality is available within the radio chip itself, there
are still many factors to consider when implementing a flexible,
general radio stack.</p>
<p>The software radio stack that drives the CC2420 radio consists of
many layers that sit between the application and hardware.  The highest
levels of the radio stack modify data and headers in each packet, while
the lowest levels determine the actual send and receive behavior.
By understanding the functionality at each layer of the stack, as well
as the architecture of a layer itself, it is possible to easily extend
or condense the CC2420 radio stack to meet project requirements.</p>
<p>Some details about the CC2420 are out of the scope of this document.
These details can be found in the CC2420 datasheet <a class="footnote-reference" href="#id11" id="id1" name="id1">[1]</a>.</p>
</div>
<div class="section">
<h1><a id="cc2420-radio-stack-layers" name="cc2420-radio-stack-layers">2. CC2420 Radio Stack Layers</a></h1>
<div class="section">
<h2><a id="layer-architecture" name="layer-architecture">2.1 Layer Architecture</a></h2>
<p>The CC2420 radio stack consists of layers of functionality stacked
on top of each other to provide a complete mechanism that
modifies, filters, transmits, and controls inbound and outbound messages.
Each layer is a distinct module that can provide and use three sets of
interfaces in relation to the actual radio stack:  Send, Receive, and
SplitControl.  If a general layer provides one of those interfaces, it
also uses that interface from the layer below it in the stack.  This
allows any given layer to be inserted anywhere in the stack through
independant wiring. For example::</p>
<pre class="literal-block">
provides interface Send;
uses interface Send as SubSend;

provides interface Receive;
uses interface Receive as SubReceive;

provides interface SplitControl;
uses interface SplitControl as subControl;
</pre>
<p>The actual wiring of the CC2420 radio stack is done at the highest level
of the stack, inside CC2420ActiveMessageC.  This configuration defines
three branches:  Send, Receive, and SplitControl. Note that not all
layers need to provide and use all three Send, Receive, and SplitControl
interfaces::</p>
<pre class="literal-block">
// SplitControl Layers
SplitControl = LplC;
LplC.SubControl -&gt; CsmaC;

// Send Layers
AM.SubSend -&gt; UniqueSendC;
UniqueSendC.SubSend -&gt; LinkC;
LinkC.SubSend -&gt; LplC.Send;
LplC.SubSend -&gt; TinyosNetworkC.Send;
TinyosNetworkC.SubSend -&gt; CsmaC;

// Receive Layers
AM.SubReceive -&gt; LplC;
LplC.SubReceive -&gt; UniqueReceiveC.Receive;
UniqueReceiveC.SubReceive -&gt; TinyosNetworkC.Receive;
TinyosNetworkC.SubReceive -&gt; CsmaC;
</pre>
<p>If another layer were to be added, CC2420ActiveMessageC would need
to be modified to wire it into the correct location.</p>
</div>
<div class="section">
<h2><a id="layer-descriptions" name="layer-descriptions">2.1 Layer Descriptions</a></h2>
<p>The layers found within this radio stack are in the following order:</p>
<ul>
<li><p class="first">ActiveMessageP:  This is the highest layer in the stack, responsible
for filling in details in the packet header and providing information
about the packet to the application level <a class="footnote-reference" href="#id12" id="id2" name="id2">[2]</a>.  Because the CC2420 radio
chip itself uses 802.15.4 headers in hardware <a class="footnote-reference" href="#id11" id="id3" name="id3">[1]</a>, it is not possible
for the layer to rearrange header bytes.</p>
</li>
<li><p class="first">UniqueSend:  This layer generates a unique Data Sequence
Number (DSN) byte for the packet header.  This byte is incremented once
per outgoing packet, starting with a pseudo-randomly generated number.
A receiver can detect duplicate packets by comparing
the source and DSN byte of a received packet with previous packets.
DSN is defined in the 802.15.4 specification <a class="footnote-reference" href="#id13" id="id4" name="id4">[3]</a>.</p>
</li>
<li><p class="first">PacketLink: This layer provides automatic retransmission functionality
and is responsible for retrying a packet transmission if no
acknowledgement was heard from the receiver.  PacketLink is
activated on a per-message basis, meaning the outgoing packet will
not use PacketLink unless it is configured ahead of time to do so.
PacketLink is most reliable when software acknowledgements are enabled
as opposed to hardware auto acknowledgements.</p>
</li>
<li><p class="first">CC2420AckLplP / CC2420NoAckLplP <a class="footnote-reference" href="#id14" id="id5" name="id5">[4]</a>:  These layers provide
asynchronous low power listening implementations.  Supporting both of them
is CC2420DutyCycleP.  The DutyCycleP component is responsible for
turning the radio on and off and performing receive checks.
After a detection occurs, DutyCycleP is hands off responsibility to
LowPowerListeningP to perform some transaction and turn the radio off
when convenient.  Low power listening transmissions are activated on
a per-message basis, and the layer will continuously retransmit the full outbound
packet until either a response from the receiver is heard or the
transmit time expires.</p>
<p>The AckLplP implementation supports acknowledgement gaps during the
low power listening packetized preamble, which allows
transmitters to stop early but penalizes receive check lengths.
AckLplP low power listening is optimal for high transmission, long
receive check interval networks.</p>
<p>The NoAckLplP implementation does not support acknowledgements during
the packetized preamble.  It continuously modulates the channel,
allowing the receiver to perform the smallest possible receive check.
NoAckLpl low power listening is effective for low transmission, short
receive check interval networks.</p>
</li>
<li><p class="first">UniqueReceive: This layer maintains a history of the source address
and DSN byte of the past few packets it has received, and helps
filter out duplicate received packets.</p>
</li>
<li><p class="first">TinyosNetworkC: This layer allows the TinyOS 2.x radio stack to
interoperate with other non-TinyOS networks.  Proposed 6LowPAN
specifications include a network identifier byte after the
standard 802.15.4 header <a class="footnote-reference" href="#id15" id="id6" name="id6">[5]</a>. If interoperability frames are
used, the dispatch layer provides functionality for setting
the network byte on outgoing packets and filtering non-TinyOS
incoming packets.</p>
</li>
<li><p class="first">CsmaC:  This layer is responsible for defining 802.15.4 FCF
byte information in the outbound packet, providing default
backoff times when the radio detects a channel in use, and
defining the power-up/power-down procedure for the radio.</p>
</li>
<li><p class="first">TransmitP/ReceiveP: These layers are responsible for interacting
directly with the radio through the SPI bus, interrupts, and GPIO lines.</p>
</li>
</ul>
<blockquote>
<table border="1" class="docutils">
<colgroup>
<col width="47%" />
<col width="53%" />
</colgroup>
<tbody valign="top">
<tr><td colspan="2">Application Layer</td>
</tr>
</tbody>
</table>
<blockquote>
<table border="1" class="docutils">
<colgroup>
<col width="47%" />
<col width="53%" />
</colgroup>
<tbody valign="top">
<tr><td colspan="2">Active Message Layer</td>
</tr>
</tbody>
</table>
<table border="1" class="docutils">
<colgroup>
<col width="47%" />
<col width="53%" />
</colgroup>
<tbody valign="top">
<tr><td colspan="2">Unique Send Layer</td>
</tr>
</tbody>
</table>
<table border="1" class="docutils">
<colgroup>
<col width="47%" />
<col width="53%" />
</colgroup>
<tbody valign="top">
<tr><td colspan="2">Optional Packet Link Layer</td>
</tr>
</tbody>
</table>
<table border="1" class="docutils">
<colgroup>
<col width="47%" />
<col width="53%" />
</colgroup>
<tbody valign="top">
<tr><td colspan="2">Optional Low Power Listening Implementations</td>
</tr>
</tbody>
</table>
<table border="1" class="docutils">
<colgroup>
<col width="47%" />
<col width="53%" />
</colgroup>
<tbody valign="top">
<tr><td colspan="2">Unique Receive Filtering Layer</td>
</tr>
</tbody>
</table>
<table border="1" class="docutils">
<colgroup>
<col width="47%" />
<col width="53%" />
</colgroup>
<tbody valign="top">
<tr><td colspan="2">Optional 6LowPAN TinyOS Network Layer</td>
</tr>
</tbody>
</table>
<table border="1" class="docutils">
<colgroup>
<col width="47%" />
<col width="53%" />
</colgroup>
<tbody valign="top">
<tr><td colspan="2">Carrier Sense Multiple Access (CSMA)</td>
</tr>
</tbody>
</table>
</blockquote>
<p>+----------+----------+   +----------+----------+
|      ReceiveP       |   |      TransmitP      |
+----------+----------+   +----------+----------+</p>
<table border="1" class="docutils">
<colgroup>
<col width="48%" />
<col width="52%" />
</colgroup>
<tbody valign="top">
<tr><td colspan="2">SPI bus, GPIO, Interrupts, Timer Capture</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
</div>
<div class="section">
<h1><a id="cc2420-packet-format-and-specifications" name="cc2420-packet-format-and-specifications">3. CC2420 Packet Format and Specifications</a></h1>
<p>The CC2420 Packet structure is defined in CC2420.h.  The default
I-Frame CC2420 header takes on the following format::</p>
<pre class="literal-block">
typedef nx_struct cc2420_header_t {
  nxle_uint8_t length;
  nxle_uint16_t fcf;
  nxle_uint8_t dsn;
  nxle_uint16_t destpan;
  nxle_uint16_t dest;
  nxle_uint16_t src;
  nxle_uint8_t network;  // optionally included with 6LowPAN layer
  nxle_uint8_t type;
} cc2420_header_t;
</pre>
<p>All fields up to 'network' are 802.15.4 specified fields, and are
used in the CC2420 hardware itself. The 'network' field is a 6LowPAN
interoperability specification <a class="footnote-reference" href="#id15" id="id7" name="id7">[5]</a> only to be included when the
6LowPAN TinyosNetwork layer is included.  The 'type' field is a
TinyOS specific field.</p>
<p>The TinyOS T-Frame packet does not include the 'network' field, nor
the functionality found in the Dispatch layer to set and check
the 'network' field.</p>
<p>No software footer is defined for the CC2420 radio.  A 2-byte
CRC byte is auto-appended to each outbound packet by the CC2420 radio
hardware itself.</p>
<p>The maximum size of a packet is 128 bytes including its headers and
CRC, which matches the 802.15.4 specifications.  Increasing the
packet size will increase data throughput and RAM consumption
in the TinyOS application, but will also increase the probability
that interference will cause the packet to be destroyed and need
to be retransmitted. The TOSH_DATA_LENGTH preprocessor variable can
be altered to increase the size of the message_t payload at
compile time <a class="footnote-reference" href="#id12" id="id8" name="id8">[2]</a>.</p>
</div>
<div class="section">
<h1><a id="csma-ca" name="csma-ca">4. CSMA/CA</a></h1>
<div class="section">
<h2><a id="clear-channel-assessment" name="clear-channel-assessment">4.1 Clear Channel Assessment</a></h2>
<p>By default, the CC2420 radio stack performs a clear channel assessment
(CCA) before transmitting.  If the channel is not clear, the radio backs
off for some short, random period of time before attempting to transmit
again.  The CC2420 chip itself provides a strobe command to transmit
the packet if the channel is currently clear.</p>
<p>To specify whether or not to transmit with clear channel assessment,
the CC2420TransmitP requests CCA backoff input through the RadioBackoff
interface on a per-message basis.  By default, each packet will be
transmitted with CCA enabled.</p>
<p>If layers above the CSMA layer wish to disable the clear channel
assessments before transmission, they must intercept the
RadioBackoff.requestCca(...) event for that message and call back
using RadioBackoff.setCca(FALSE).</p>
</div>
<div class="section">
<h2><a id="radio-backoff" name="radio-backoff">4.2 Radio Backoff</a></h2>
<p>A backoff is a period of time where the radio pauses before attempting
to transmit. When the radio needs to backoff, it can choose one of three
backoff periods:  initialBackoff, congestionBackoff, and lplBackoff.
These are implemented through the RadioBackoff interface, which signals
out a request to specify the backoff period.  Unlike the CsmaBackoff
interface, components that are interested in adjusting the backoff can
call back using commands in the RadioBackoff interface.  This allows
multiple components to adjust the backoff period for packets they are
specifically listening to adjust.  The lower the backoff period, the
faster the transmission, but the more likely the transmitter is to hog
the channel.  Also, backoff periods should be as random as possible
to prevent two transmitters from sampling the channel at the same
moment.</p>
<p>InitialBackoff is the shortest backoff period, requested on the first
attempt to transmit a packet.</p>
<p>CongestionBackoff is a longer backoff period used when the channel is
found to be in use.  By using a longer backoff period in this case,
the transmitter is less likely to unfairly tie up the channel.</p>
<p>LplBackoff is the backoff period used for a packet being delivered
with low power listening.  Because low power listening requires
the channel to be modulated as continuously as possible while avoiding
interference with other transmitters, the low power listening
backoff period is intentionally short.</p>
</div>
</div>
<div class="section">
<h1><a id="acknowledgements" name="acknowledgements">5. Acknowledgements</a></h1>
<div class="section">
<h2><a id="hardware-vs-software-acknowledgements" name="hardware-vs-software-acknowledgements">5.1 Hardware vs. Software Acknowledgements</a></h2>
<p>Originally, the CC2420 radio stack only used hardware generated
auto-acknowledgements provided by the CC2420 chip itself.  This led
to some issues, such as false acknowledgements where the radio chip
would receive a packet and acknowledge its reception and the
microcontroller would never actually receive the packet.</p>
<p>The current CC2420 stack uses software acknowledgements, which
have a higher drop percentage. When used with the UniqueSend
and UniqueReceive interfaces, dropped acknowledgements are more desirable
than false acknowledgements.  Received packets are always acknowledged
before being filtered as a duplicate.</p>
<p>Use the PacketAcknowledgements or PacketLink interfaces
to determine if a packet was successfully acknowledged.</p>
</div>
<div class="section">
<h2><a id="data-sequence-numbers-uniquesend-and-uniquereceive" name="data-sequence-numbers-uniquesend-and-uniquereceive">5.2 Data Sequence Numbers - UniqueSend and UniqueReceive</a></h2>
<p>The 802.15.4 specification identifies a Data Sequence Number (DSN)
byte in the message header to filter out duplicate packets <a class="footnote-reference" href="#id13" id="id9" name="id9">[3]</a>.</p>
<p>The UniqueSend interface at the top of the CC2420 radio stack is
responsible for setting the DSN byte.  Upon boot, an initial DSN
byte is generated using a pseudo-random number generator with a
maximum of 8-bits (256) values.  This number is incremented on
each outgoing packet transmission.  Even if lower levels such as
PacketLink or LowPowerListening retransmit the packet, the DSN byte
stays the same for that packet.</p>
<p>The UniqueReceive interface at the bottom of the CC2420 radio stack
is responsible for filtering out duplicate messages based on
source address and DSN.  The UniqueReceive interface is not meant
to stop sophisticated replay attacks.  '</p>
<p>As packets are received, DSN and source address information is placed
into elements of an array. Each subsequent message from previously
logged addresses overwrite information in the element allocated to
that source address. This prevents UniqueReceive's history from
losing DSN byte information from sources that are not able to access
the channel as often. If the number of elements in the history array
runs out, UniqueReceive uses a best effort method to avoid replacing
recently updated DSN/Source information entries.</p>
</div>
</div>
<div class="section">
<h1><a id="packetlink-implementation" name="packetlink-implementation">6. PacketLink Implementation</a></h1>
<p>PacketLink is a layer added to the CC2420 radio stack to help unicast
packets get delivered successfully.  In previous version of TinyOS radio
stacks, it was left up to the application layer to retry a message
transmission if the application determined the message was not properly
received.  The PacketLink layer helps to remove the reliable delivery
responsibility and functional baggage from application layers.</p>
<div class="section">
<h2><a id="compiling-in-the-packetlink-layer" name="compiling-in-the-packetlink-layer">6.1 Compiling in the PacketLink layer</a></h2>
<p>Because the PacketLink layer uses up extra memory footprint,
it is not compiled in by default.  Developers can simply define
the preprocessor variable PACKET_LINK to compile the functionality
of the PacketLink layer in with the CC2420 stack.</p>
</div>
<div class="section">
<h2><a id="implementation-and-usage" name="implementation-and-usage">6.2 Implementation and Usage</a></h2>
<p>To send a message using PacketLink, the PacketLink
interface must be called ahead of time to specify two fields in the outbound
message's metadata::</p>
<pre class="literal-block">
command void setRetries(message_t *msg, uint16_t maxRetries);
command void setRetryDelay(message_t *msg, uint16_t retryDelay);
</pre>
<p>The first command, setRetries(..), will specify the maximum number
of times the message should be sent before the radio stack stops
transmission.  The second command, setRetryDelay(..), specifies
the amount of delay in milliseconds between each retry.  The combination
of these two commands can set a packet to retry as many times as needed
for as long as necessary.</p>
<p>Because PacketLink relies on acknowledgements, false acknowledgements
from the receiver will cause PacketLink to fail.  If using software
acknowledgements, false acknowledgements can still occur as a result
of the limited DSN space, other 802.15.4 radios in the area with
the same destination address, etc.</p>
</div>
</div>
<div class="section">
<h1><a id="asynchronous-low-power-listening-implementation" name="asynchronous-low-power-listening-implementation">7. Asynchronous Low Power Listening Implementation</a></h1>
<p>Because the Low Power Listening layer uses up extra memory footprint,
it is not compiled in by default.  Developers can simply define
the preprocessor variable LOW_POWER_LISTENING to compile the functionality
of the Low Power Listening layer in with the CC2420 stack.</p>
<div class="section">
<h2><a id="design-considerations" name="design-considerations">7.1 Design Considerations</a></h2>
<p>The CC2420 radio stack low power listening implementation relies
on clear channel assessments to determine if there is a transmitter
nearby.  This allows the receiver to turn on and determine there are no
transmitters in a shorter amount of time than leaving the radio on
long enough to pick up a full packet.</p>
<p>The transmitters perform a message delivery by transmitting
the full packet over and over again for twice the duration of the receiver's
duty cycle period.  Transmitting for twice as long increases the
probability that the message will be detected by the receiver, and
allows the receiver to shave off a small amount of time it needs to
keep its radio on.</p>
<p>Typically, the transmission of a single packet takes on the following
form over time:</p>
<blockquote>
<table border="1" class="docutils">
<colgroup>
<col width="42%" />
<col width="21%" />
<col width="38%" />
</colgroup>
<tbody valign="top">
<tr><td>LPL Backoff</td>
<td>Packet Tx</td>
<td>Ack Wait</td>
</tr>
</tbody>
</table>
</blockquote>
<p>To decrease the amount of time required for a receive check, the channel
must be modulated by the transmitter as continuously as possible.
The only period where the channel is modulated is during the
Packet Transmission phase.  The receiver must continuosly sample the CCA pin
a moment longer than the LPL Backoff period and Ack Wait period combined
to overlap the Packet Transmission period.  By making the LPL backoff
period as short as possible, we can decrease the amount of time a receiver's
radio must be turned on when performing a receive check.</p>
<p>If two transmitters attempt to transmit using low power listening,
one transmitter may hog the channel if its LPL backoff period
is set too short.  Both nodes transmitting at the same time
will cause interference and prevent each other from
successfully delivering their messages to the intended recipient.</p>
<p>To allow multiple transmitters to transmit low power listening packets
at the same time, the LPL backoff period needed to be increased
greater than the desired minimum.  This increases the amount of time
receiver radios need to be on to perform a receive check because
the channel is no longer being modulated as continuously as possible.
In other words, the channel is allowed to be shared amongst multiple
transmitters at the expense of power consumption.</p>
</div>
<div class="section">
<h2><a id="minimizing-power-consumption" name="minimizing-power-consumption">7.2 Minimizing Power Consumption</a></h2>
<p>There are several methods the CC2420 radio stack uses to minimize
power consumption:</p>
<ol class="arabic simple">
<li>Invalid Packet Shutdown</li>
</ol>
<blockquote>
Typically, packets are filtered out by address at the radio hardware
level.  When a receiver wakes up and does not receive any
packets into the low power listening layer of the radio stack, it
will automatically go back to sleep after some period of time.  As a
secondary backup, if address decoding on the radio chip is disabled,
the low power listening implementation will shut down the radio if
three packets are receive that do not belong to the node.  This helps
prevent against denial of sleep attacks or the typical transmission
behavior found in an ad-hoc network with many nodes.</blockquote>
<ol class="arabic simple" start="2">
<li>Early Transmission Completion</li>
</ol>
<blockquote>
A transmitter typically sends a packet for twice the amount of time
as the receiver's receive check period.  This increases the probability
that the receiver will detect the packet.  However, if the transmitter receives
an acknowledgement before the end of its transmission period, it
will stop transmitting to save energy.  This is an improvement
over previous low power listening implementations, which transmitted
for the full period of time regardless of whether the receiver has
already woken up and received the packet.</blockquote>
<ol class="arabic simple" start="3">
<li>Auto Shutdown</li>
</ol>
<blockquote>
If the radio does not send or receive messages for some period of
time while low power listening is enabled, the radio will automatically
turn off and begin duty cycling at its specified duty cycle period.</blockquote>
<ol class="arabic simple" start="4">
<li>CCA Sampling Strategy</li>
</ol>
<blockquote>
The actual receive check is performed in a loop inside a function,
not a spinning task.  This allows the sampling to be performed
continuously, with the goal of turning the radio off as quickly as
possible without interruption.</blockquote>
</div>
</div>
<div class="section">
<h1><a id="cc2420-settings-and-registers" name="cc2420-settings-and-registers">8. CC2420 Settings and Registers</a></h1>
<p>To interact with registers on the CC2420 chip, the SPI bus must be
acquired, the chip selecct (CSn) pin must be cleared, and then the
interaction may occur.  After the interaction completes, the
CSn pin must be set high.</p>
<p>All registers and strobes are defined in the CC2420.h file, and most
are accessible through the CC2420SpiC component.  If your application
requires access to a specific register or strobe, the CC2420SpiC component
is the place to add access to it.</p>
<p>Configuring the CC2420 requires the developer to access the CC2420Config
interface provided by CC2420ControlC.  First call the CC2420Config commands to
change the desired settings of the radio. If the radio happens to
be off, the changes will be committed at the time it is turned on.
Alternatively, calling sync() will commit the changes to the CC2420.</p>
<p>RSSI can be sampled directly by calling the ReadRssi interface provided
by CC2420ControlC.  See page 50 of the CC2420 datasheet for information
on how to convert RSSI to LQI and why it may not be such a good idea <a class="footnote-reference" href="#id11" id="id10" name="id10">[1]</a>.</p>
</div>
<div class="section">
<h1><a id="cross-platform-portability" name="cross-platform-portability">9. Cross-platform Portability</a></h1>
<p>To port the CC2420 radio to another platform, the following interfaces
need to be implemented::</p>
<pre class="literal-block">
// GPIO Pins
interface GeneralIO as CCA;
interface GeneralIO as CSN;
interface GeneralIO as FIFO;
interface GeneralIO as FIFOP;
interface GeneralIO as RSTN;
interface GeneralIO as SFD;
interface GeneralIO as VREN;

// SPI Bus
interface Resource;
interface SpiByte;
interface SpiPacket;

// Interrupts
interface GpioCapture as CaptureSFD;
interface GpioInterrupt as InterruptCCA;
interface GpioInterrupt as InterruptFIFOP;
</pre>
<p>The GpioCapture interface is tied through the Timer to provide a relative time
at which the interrupt occurred.  This is useful for timestamping received
packets for node synchronization.</p>
<p>If the CC2420 is not connected to the proper interrupt lines,
interrupts can be emulated through the use of a spinning task
that polls the GPIO pin.  The MICAz implementation, for example, does this
for the CCA interrupt.</p>
</div>
<div class="section">
<h1><a id="future-improvement-recommendations" name="future-improvement-recommendations">10. Future Improvement Recommendations</a></h1>
<p>Many improvements can be made to the CC2420 stack.  Below are some
recommendations:</p>
<div class="section">
<h2><a id="aes-encryption" name="aes-encryption">10.1 AES Encryption</a></h2>
<p>The CC2420 chip itself provides AES-128 encryption. The implementation
involves loading the security RAM buffers on the CC2420 with the information
to be encrypted - this would be the payload of a packet, without
the header.  After the payload is encrypted, the microcontroller reads
out of the security RAM buffer and concatenates the data with the
unencrypted packet header.  This full packet would be uploaded again to the CC2420
TXFIFO buffer and transmitted.</p>
<p>Because the CC2420 cannot begin encryption at a particular offset
and needs to be written, read, and re-written, use of the AES-128 may be
inefficient and will certainly decrease throughput.</p>
</div>
<div class="section">
<h2><a id="authentication" name="authentication">10.2 Authentication</a></h2>
<p>In many cases, authentication is more desirable than encryption.
Encryption significantly increases energy and decreases packet throughput,
which does not meet some application requirements. A layer could be
developed and added toward the bottom of the radio stack that validates
neighbors, preventing packets from invalid neighbors from reaching the
application layer.  Several proprietary authentication layers have
been developed for the CC2420 stack, but so far none are available to
the general public.</p>
<p>A solid authentication layer would most likely involve the use of a
neighbor table and 32-bit frame counts to prevent against replay attacks.
Once a neighbor is verified and established, the node needs to ensure that
future packets are still coming from the same trusted source.  Again,
some high speed low energy proprietary methods to accomplish this exist, but
encryption is typically the standard method used.</p>
</div>
<div class="section">
<h2><a id="synchronous-low-power-listening" name="synchronous-low-power-listening">10.3 Synchronous Low Power Listening</a></h2>
<p>A synchronous low power listening layer can be transparently built on
top of the asynchronous low power listening layer.  One implementation
of this has already been done on a version of the CC1000 radio stack.
Moteiv's Boomerang radio stack also has a synchronous low power listening
layer built as a standalone solution.</p>
<p>In the case of building a synchronous layer on top of the asynchronous
low power listening layer, a transmitter's radio stack can detect when
a particular receiver is performing its receive checks by verifying the
packet was acknowledged after a sendDone event.  The transmitter can then
build a table to know when to begin transmission for that particular receiver.
Each successful transmission would need to adjust the table with updated
information to avoid clock skew problems.</p>
<p>The asynchronous low power listening stack needs to be altered a bit
to make this strategy successful.  Currently, duty cycling is started
and stopped as packets are detected, received, and transmitted.  The
stack would need to be altered to keep a constant clock running in the
background that determines when to perform receive checks.  The
clock should not be affected by normal radio stack Rx/Tx behavior.  This
would allow the receiver to maintain a predictable receive check cycle
for the transmitter to follow.</p>
<p>If the synchronous low power listening layer loses synchronization,
the radio stack can always fall back on the asynchronous low power listening
layer for successful message delivery.</p>
</div>
<div class="section">
<h2><a id="neighbor-tables" name="neighbor-tables">10.4 Neighbor Tables</a></h2>
<p>Moteiv's Boomerange Sensornet Protocol (SP) implementation is a
good model to follow for radio stack architecture.  One of the nice features
of SP is the design and implementation of the neighbor table.  By
providing and sharing neighbor table information across the entire
CC2420 radio stack, RAM can be conserved throughout the radio stack
and TinyOS applications.</p>
</div>
<div class="section">
<h2><a id="radio-independant-layers" name="radio-independant-layers">10.5 Radio Independant Layers</a></h2>
<p>The best radio stack architecture is one that is completely radio independant.
Many of the layers in the CC2420 stack can be implemented independant
of the hardware underneath if the radio stack architecture was redesigned
and reimplemented. The low power listening receive check strategy may need a
hardware-dependant implementation, but other layers like PacketLink,
UniqueSend, UniqueReceive, ActiveMessage, Dispatch, etc. do not require
a CC2420 underneath to operate properly.  The ultimate TinyOS radio
stack would be one that forms an abstraction between radio-dependant
layers and radio-independant layers, and operates with the same
behavior across any radio chip.</p>
</div>
<div class="section">
<h2><a id="extendable-metadata" name="extendable-metadata">10.6 Extendable Metadata</a></h2>
<p>Layers added into the radio stack may require extra bytes of metadata.
The PacketLink layer, for example, requires two extra fields
in each message's metadata to hold the message's max retries and
delay between retries.  The low power listening layer requires
an extra field to specify the destination's duty cycle period for
a proper delivery.</p>
<p>If layers are not included in the radio stack during compile time,
their fields should not be included in the message_t's metadata.</p>
<p>One version of extendable metadata was implementing using an array at the end
of the metadata struct that would adjust its size based on which layers were
compiled in and what size fields they required.  A combination of
compiler support in the form of unique(..) and uniqueCount(..) functions
made it possible for the array to adjust its size.</p>
<p>Explicit compiler support would be the most desirable solution to add
fields to a struct as they are needed.</p>
</div>
<div class="section">
<h2><a id="error-correcting-codes-ecc" name="error-correcting-codes-ecc">10.7 Error Correcting Codes (ECC)</a></h2>
<p>When two nodes are communicating near the edge of their RF range,
it has been observed that interference may cause the packet to be
corrupted enough that the CRC byte and payload actually passes
the check, even though the payload is not valid.  There is a one
in 65535 chance of a CRC byte passing the check for a corrupted
packet.  Although this is slim, in many cases it is unacceptable.
Some work arounds have implemented an extra byte of software generated
CRC to add to the reliability, and tests have proven its effectiveness.
Taking this a step further, an ECC layer in the radio stack would help
correct corrupted payloads and increase the distance at which nodes
can reliably communicate.</p>
</div>
</div>
<div class="section">
<h1><a id="author-s-address" name="author-s-address">11. Author's Address</a></h1>
<div class="line-block">
<div class="line">David Moss</div>
<div class="line">Rincon Research Corporation</div>
<div class="line">101 N. Wilmot, Suite 101</div>
<div class="line">Tucson, AZ  85750</div>
<div class="line"><br /></div>
<div class="line">phone - +1 520 519 3138</div>
<div class="line">email ? <a class="reference" href="mailto:dmm&#64;rincon.com">dmm&#64;rincon.com</a></div>
<div class="line"><br /></div>
<div class="line"><br /></div>
<div class="line">Jonathan Hui</div>
<div class="line">657 Mission St. Ste. 600</div>
<div class="line">Arched Rock Corporation</div>
<div class="line">San Francisco, CA 94105-4120</div>
<div class="line"><br /></div>
<div class="line">phone - +1 415 692 0828</div>
<div class="line">email - <a class="reference" href="mailto:jhui&#64;archedrock.com">jhui&#64;archedrock.com</a></div>
<div class="line"><br /></div>
<div class="line"><br /></div>
<div class="line">Philip Levis</div>
<div class="line">358 Gates Hall</div>
<div class="line">Stanford University</div>
<div class="line">Stanford, CA 94305-9030</div>
<div class="line"><br /></div>
<div class="line">phone - +1 650 725 9046</div>
<div class="line">email - <a class="reference" href="mailto:pal&#64;cs.stanford.edu">pal&#64;cs.stanford.edu</a></div>
<div class="line"><br /></div>
<div class="line"><br /></div>
<div class="line">Jung Il Choi</div>
<div class="line">&lt;contact&gt;</div>
<div class="line">phone -</div>
<div class="line">email -</div>
</div>
</div>
<div class="section">
<h1><a id="citations" name="citations">12. Citations</a></h1>
<table class="docutils footnote" frame="void" id="id11" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a name="id11">[1]</a></td><td><em>(<a class="fn-backref" href="#id1">1</a>, <a class="fn-backref" href="#id3">2</a>, <a class="fn-backref" href="#id10">3</a>)</em> TI/Chipcon CC2420 Datasheet.  <a class="reference" href="http://www.chipcon.com/files/CC2420_Data_Sheet_1_3.pdf">http://www.chipcon.com/files/CC2420_Data_Sheet_1_3.pdf</a></td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="id12" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a name="id12">[2]</a></td><td><em>(<a class="fn-backref" href="#id2">1</a>, <a class="fn-backref" href="#id8">2</a>)</em> TEP111: message_t</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="id13" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a name="id13">[3]</a></td><td><em>(<a class="fn-backref" href="#id4">1</a>, <a class="fn-backref" href="#id9">2</a>)</em> IEEE 802.15.4 Specification: <a class="reference" href="http://standards.ieee.org/getieee802/802.15.html">http://standards.ieee.org/getieee802/802.15.html</a></td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="id14" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id5" name="id14">[4]</a></td><td>TEP105: Low Power Listening</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="id15" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a name="id15">[5]</a></td><td><em>(<a class="fn-backref" href="#id6">1</a>, <a class="fn-backref" href="#id7">2</a>)</em> TEP125: TinyOS 802.15.4 Frames</td></tr>
</tbody>
</table>
</div>
</div>
</body>
</html>
